Chromosomes, when paired, occur as both autosomes and allosomes. A pair of autosomes are identical in form, size and behavior. This is not necessarily the case for allosomes. Whether the allosomes in an organism are homozygous, heterozygous, diploid or haploid is typically used to determine the sex of that organism.
In the absence of allosomes and environmental factors, what mechanism(s) would allow autosomes alone to determine the sex of an organism?
EDIT: There are two such autosomal mechanisms of which I am aware exist on Earth.
- Sex chromosomes are speculated to have evolved from chromosomes with a sex-determining gene. Phenotype was determined by the presence of a dominant allele. Later the chromosomes containing the dominant allele inverted and became virtually unable to crossover with their homologue. If this allele resulted in males, it gave rise to the XY system; if females, the ZW system. Some animals still use alleles, so lack true allosomes because these chromosomes may crossover.
- Fungi do not have sexes because all individuals produce the same type of gamete. Reproductive compatibility is determined by mating types which, as the name implies, work similarly to blood types: if two individuals share the same mating type, they cannot reproduce. This mechanism is not applicable to organisms with sexes.